Method for producing acrylic acid

ABSTRACT

A process for the fractional condensation of a hot gas mixture which as well as acrylic acid contains at least one further condensable component in a column in the presence of at least one stabilizer comprises metering in at least a portion of the at least one stabilizer as a melt.

The present invention relates to a process for fractional condensationof a hot gas mixture comprising acrylic acid.

DE-A 197 40 253 and German patent application number 102 35 847.8 ofAug. 5, 2002 disclose that a reaction mixture from the catalytic gasphase oxidation to give acrylic acid can be fractionally condensed bypassing it upward into a column having separating internals andcondensing out the condensable components by cooling.

In the latter process (FIG. 1), preference is given to stabilizing thecolumn in the following way:

The upper column region is stabilized by a water-soluble phenoliccompound, preferably hydroquinone monomethyl ether. Stabilizer is addedas a solution in dilute acid into the reflux stream or into the quenchcircuit.

The remaining column region is stabilized by a 0.1–1% by weight solutionof phenothiazine in acrylic acid which is added in the column regionwhere the acrylic acid concentration is 5–15% and the waterconcentration is 80–95%. The quantity added is determined in such a waythat the phenothiazine contents in the acrylic acid fraction(medium-boiler fraction, stream 7) is 10–1000 ppm, preferably 50–500ppm.

A disadvantage of the process described there is that the water-solublephenolic compounds, in particular hydroquinone monomethyl ether, onlyhave limited solubility in the dilute acid which occurs as thelow-boiler fraction and is used as the solvent. This results in largequantities of stabilizer solution being required.

A further disadvantage is that the effective but sparingly water-solublestabilizer phenothiazine cannot be metered into the upper column regionowing to the high water concentration there. Only the dilute acid oracrylic acid are useful as solvents for phenothiazine. Other solventscarry extraneous materials into the system and are thereforeundesirable. Phenothiazine is virtually insoluble in dilute acid. Ifacrylic acid were to be used as solvent, the acrylic acid used would belost in the dilute acid.

It is an object of the present invention to provide a process forpreparing acrylic acid in which phenothiazine can also be used as astabilizer in regions of high water concentration.

We have found that this object is achieved by a process for thefractional condensation of a hot gas mixture which as well as acrylicacid contains at least one further condensable component in a column inthe presence of at least one stabilizer, which comprises metering in atleast a portion of the at least one stabilizer as a melt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a customary embodiment of fractional condensation of a hotgas mixture comprising acrylic acid wherein line 1 is cooled in a quench(spray cooler) IV and fed via line 2 to the bottom region I.a of thecolumn. The coolant (unvaporized high-boiler fraction from I.a) isrecycled into the quench (spray cooler) IV via line 3 in order to coolthe hot gas mixture.

FIG. 2 shows an embodiment of fractional condensation of a hot gasmixture comprising acrylic acid wherein the high-boiler fractions of thecolumn from sections I.a and I.b may be recycled separately into thequench. In this embodiment the fraction from I.b is directly, withoutheat exchangers, recycled to the spray cooler apparatus IV.

FIG. 3 shows the embodiment of FIG. 2, but in the absence of heatexchanger III.

FIG. 4 shows the embodiment of FIG. 1, but in the absence of heatexchanger III.

FIG. 5 shows an embodiment of fractional condensation of a hot gasmixture comprising acrylic acid wherein cooling may take place isolatedfrom the column I in a separate apparatus represented by heat exchangerII and external quench (spray cooler) V.

The process according to the invention is generally carried out asfollows:

Useful hot gas mixtures include such gas mixtures as the reaction gasmixture resulting from the catalytic gas phase oxidation of C₃-alkanes,-alkenes, -alkanols and/or -alkanals and/or precursors thereof to giveacrylic acid by known processes. Propene, propane and acrolein areparticularly advantageously used for preparing acrylic acid. However,useful starting compounds for acrylic acid also include those from whichthe actual C₃ starting compound is first formed during the gas phaseoxidation as an intermediate. Acrylic acid may be prepared directly frompropane. When propane is used as the starting material, it may beconverted by known catalytic oxidehydrogenation, homogeneousoxidehydrogenation or catalytic dehydrogenation processes to give apropene/propane mixture. Useful propene/propane mixtures also includerefinery propene (about 70% propene and 30% propane) or cracker propene(about 95% propene and 5% propane). When a propene/propane mixture isused for preparing acrylic acid, propane acts as a diluent gas and/orreactant. The preparation of acrylic acid is generally carried out bydiluting the starting gases with gases which are inert under the chosenreaction conditions such as nitrogen (N₂), CO₂, saturatedC₁–C₆-hydrocarbons and/or steam and passing them in the mixture withoxygen (O₂) or an oxygen-containing gas at elevated temperatures(customarily from 200 to 450° C.) and also optionally elevated pressureover transition metal (e.g. Mo-, V-, W- and/or Fe-containing) mixedoxide catalysts and converting them oxidatively into acrylic acid. Thesereactions are carried out, for example, in one or more steps.

The resulting reaction gas mixture, in addition to the desired acid,contains secondary components such as unconverted acrolein and/orpropene, steam, carbon monoxide, carbon dioxide, nitrogen, oxygen,acetic acid, propionic acid, formaldehyde, further aldehydes and maleicacid or maleic anhydride. Customarily, the reaction gas mixturecomprises, based in each case on the total reaction gas mixture, from 1to 30% by weight of acrylic acid, from 0.01 to 1% by weight of propeneand from 0.05 to 1% by weight of acrolein, from 0.05 to 10% by weight ofoxygen, from 0.01 to 3% by weight of acetic acid, from 0.01 to 2% byweight of propionic acid, from 0.05 to 1% by weight of formaldehyde,from 0.05 to 2% by weight of other aldehydes, from 0.01 to 0.5% byweight of maleic acid and maleic anhydride and the remainder comprisesinert diluent gases. The inert diluent gases include in particularsaturated C₁–C₆-hydrocarbons, such as methane and/or propane, and alsosteam, carbon oxides and nitrogen. Thus, such a gas mixture comprises,in addition to the target component acrylic acid, which condensespredominantly as the medium-boiler fraction, further compounds in thehigh-boiler and low-boiler range and also noncondensable fractions. Themedium-boiler fraction essentially consists of the components whichhave, at atmospheric pressure, a boiling point in the temperatureinterval of, for example, from 120 to 180° C. in the case of acrylicacid, in particular in the range of +/−10° C. around that of the productof value, i.e. from about 131 to 151° C. for acrylic acid.

The reaction gas mixture may be cooled indirectly, for example usingheat exchangers, which are known per se to those skilled in the art andare subject to no restriction, or directly, for example using a quench,and preferably by direct cooling.

This may be effected either in the bottom region of the column or elseisolated from the column in a separate apparatus IV as shown in FIGS. 1to 5. In this case, the hot gas mixture to be condensed having atemperature of from 200 to 400° C. from line 1 is customarily cooled ina quench IV to a temperature of from 100 to 180° C. and fed via line 2to the bottom region I.a of the column. The coolant (unvaporizedhigh-boiler fraction from I.a) is recycled into the quench IV via line 3in order to cool the hot gas mixture.

In a particular embodiment, the high-boiler fractions from I.a and I.bmay be recycled separately into the quench (FIG. 2, stream 3 and stream3 a). Particular preference is given to feeding the high-boiler fractionfrom I.b directly, without heat exchangers, to the apparatus IV (FIG. 2,stream 3 a).

A portion of the coolant from the circuit III/IV, customarily from 0.5to 5% by weight based on 100% by weight of condensate in the sidestream,may be discharged from the process (stream 4). In the case of acrylicacid preparation, this stream generally has the following composition:

10–40% by weight of acrylic acid 10–40% by weight of diacrylic acid 5–15% by weight of maleic acid/maleic anhydride  1–3% by weight ofbenzoic acid  2–6% by weight of phthalic acid/phthalic anhydrideremainder: stabilizers, polymeric acrylic acid, higher molecular weightMichael addition products of acrylic acid (tri, tetraacrylic acid, etc.)

Discharge may refer to disposal, for example by incineration, or thedischarged stream may be subjected, for example, to a thermal and/orcatalytic treatment, for example, for the purpose of dissociating thehigh-boilers, whose removed dissociation products may be fed back intothe process according to the invention at any desired point.

Useful quenching devices IV include all devices for this purpose knownfrom the prior art (for example, spray coolers, Venturi scrubbers,bubble columns or other apparatus having sprayed surfaces), andpreference is given to using Venturi scrubbers or spray coolers.

Indirect cooling or heating of the quenching liquid may be effectedusing any conventional heat transferor or heat exchanger. Preference isgiven to tube bundle heat exchangers, plate heat exchangers or aircoolers. The temperature of the quenching liquid after leaving the heatexchanger III is normally from 70 to 200° C., frequently from 100 to150° C. Useful coolants include air for an appropriate air cooler andcooling liquids, in particular water, for other cooling devices.

Depending on the separation task, it may also be possible to do withoutthe heat exchanger III (FIG. 3 and FIG. 4).

The cooled product gas mixture which is passed into the lowermost regionIa of a distillation column equipped with internals is separated byfractional condensation into one or more low-boiler, medium-boiler andhigh-boiler fractions, which are removed via sidestream takeoffs on therespective column sections.

The operating pressure in the column is generally from 0.5 to 5 bar(absolute), frequently from 0.5 to 3 bar (absolute) and in many casesfrom 0.5 to 2 bar (absolute).

Useful column internals include in principle all conventional internals,in particular trays, packings and/or bubbled packings. Among the trays,preference is given to bubble cap trays, sieve trays, valve trays,Thormann trays and/or dual flow trays. Typically, the total number oftrays in a tray column is from 20 to 80, preferably from 50 to 80.

The possibility also exists of adding further cooling circuits to thecolumn. To this end, liquid is withdrawn from the column by means of acollecting tray, this liquid is cooled by means of a suitable heatexchanger and cooled liquid is fed back into the column above thetakeoff point (not shown in the figures).

It will be appreciated that a suitable diluent may also be used in theheat exchanger/cooling quench system III/IV. Useful diluents includepolar solvents which are inert under the reaction conditions having aboiling point and melting point between 180° C. and 320° C. atatmospheric pressure. Preference is given to ethylhexanoic acid,diphenyl ether, nonylphenol and dibutylformamide. The diluent can berecovered by distillative workup of the discharged high-boiler fraction(stream 4) and recycled. If necessary, commercial dispersing assistantsmay be added to the high-boiler fraction in the heat exchanger/coolingquench system III/IV. Useful dispersing assistants may be anionic,cationic or nonionic.

In the lowermost column region (Ia, the region below the high-boilertakeoff), there is virtually no condensation of the high-boilers (excepton any cooled bridges).

The discharged quenching liquid (stream 4) can be advantageouslysubjected to thermal and/or catalytic dissociation to dissociate thoseproducts of value that are dissociable back to their starting materials,for example acrylic acid oligomer to acrylic acid monomer, in a knownmanner. The dissociation products are advantageously introduced backinto the quench or the column.

Preference is given to subjecting the discharged quenching liquid(stream 4) to thermal treatment. The thermal treatment is usuallycarried out at from 170 to 190° C. and a pressure of from 300 to 900mbar. The residence time is typically from 1 to 10 hours. Thedissociation products, after removal, are advantageously introduced backinto the quench or the column. The residence time is preferablydetermined by the viscosity of the dissociation residue so that thedissociation residue remains pumpable at the output temperature.Particular preference is given to discharging the dissociation residueat regular intervals and diluting it with from 10 to 30% of a suitablediluent, for example methanol. The pour point of the diluteddissociation residue is customarily from 20 to 50° C.

The crude acrylic acid withdrawn as a medium-boiler in the sidestream(stream 5) generally comprises, as well as acrylic acid,

from 0.1 to 2% by weight of lower carboxylic acids, for example aceticacid from 0.5 to 5% by weight of water from 0.05 to 1% by weight of lowmolecular weight aldehydes from 0.01 to 1% by weight of maleic acidand/or maleic anhydride from 1 to 500 ppm by weight of stabilizer,based in each case on the weight of the crude acrylic acid.

The crude acrylic acid withdrawn as a medium-boiler fraction (stream 5)may either be esterified directly or, for the purposes of furtherpurification, fed to a crystallization step while, in this case, theresulting mother liquor is advantageously fed back into the column asreflux (below the takeoff of the medium-boiler fraction, stream 6).

Such a crystallization step is generally carried out without addition ofa solvent, in particular without addition of an organic solvent. Thecrystallization process to be employed is subject to no restriction. Thecrystallization may be carried out continuously or batchwise, in one ormore steps to almost any desired degree of purity. If required, watermay be added prior to crystallization to the crude acrylic acid to bepurified by crystallization (up to 10% by weight or more, preferably upto 5% by weight, based on the amount of acrylic acid present). Suchaddition facilitates the removal of low carboxylic acids, e.g. aceticacid, which are contained in the crude acrylic acid as a by-product,since these are incorporated into the acrylic acid crystals to a slightextent in the presence of water. Also, the presence of water reduces thetendency to encrustation in the crystallizer.

Depending on the separation task, the possibility also exists of using aportion of the crude acrylic acid withdrawn as the medium-boilerfraction as an additional reflux in column section I.b (stream 6).

The low-boiler fraction of the column may be cooled indirectly, forexample using heat exchangers, which are known per se to those skilledin the art and are subject to no restriction, or directly, for exampleusing a quench, and preferably by direct cooling.

The cooling may take place isolated from the column in a separateapparatus (see FIG. 5) or else in the top region of column I.d, as shownin FIG. 1. In this case, the condensed low-boiler fraction from columnregion I.d (stream 7) is generally introduced into the cooler II at atemperature of from 50 to 100° C.

Indirect cooling of the condensed low-boiler fraction may be effectedusing any conventional heat transferor or heat exchanger. Preference isgiven to tube bundle heat exchangers, plate heat exchangers or aircoolers. The temperature of the condensed low-boiler fraction afterleaving the heat exchanger II is normally from 20 to 60° C., frequentlyfrom 20 to 35° C. Useful coolants include air for an appropriate aircooler and cooling liquids, in particular water, for other coolingdevices.

It will be appreciated that for the purposes of an integrated heatsystem, the heat of condensation of the low-boiler fraction may even beused completely or partially for melting acrylic acid crystals in acrystallization step or for vaporizing liquid propene upstream of thereactors to produce the hot, acrylic acid-containing gas mixture.

The low-boiler fraction (stream 7, dilute acid) may be partiallyrecycled to the top of the column (stream 8), partially used as refluxfor the column section I.c (stream 9) and partially discharged (stream10). In the case of acrylic acid preparation, it generally comprises:

from 80–95% by weight of water from 2–15% by weight of acetic acid from1–5% by weight of acrylic acid from 0.05–1% by weight of lower aldehydes(e.g. acrolein, formaldehyde)

The portion of low-boiler fraction which is recycled to the top of thecolumn is customarily controlled in such a way that uniform gas andliquid loading of the separating internals of the column and optimalmaterial separation are ensured. Preference is given to using “modal”temperature control: for this purpose, three temperature measuringpoints (giving the corresponding temperatures T1, T2 and T3) areinstalled in the region of a marked temperature or concentration jump. Avalue obtained from the three temperature measuring points, for example(T1−2×T2+T3), may be used as the quantity for the column reflux. Also,conductivity control is possible instead of “modal” temperature control.

Advantageously, the low-boilers may also be condensed in an externalquench V (FIG. 5) which is operated with dilute acid. In principle, thisquenching system may also be configured as described for quenchingsystem IV.

The uncondensable constituents of the product gas mixture (nitrogen,oxygen, propane, propene, or isobutane, isobutene, carbon monoxide,carbon dioxide, etc.) are discharged at the top of the column (stream11) or preferably, optionally after purification, at least partiallyrecycled into the gas phase oxidation as circuit gas. Particularpreference is given to heating stream 11 to from 4 to 10° C. above thetop temperature of the distillation column which prevents possiblecondensation in the offgas or circuit gas pipes.

The temperature at the base of the column is typically from 90 to 130°C., whereas the top temperature is normally from 50 to 100° C.,frequently from 60 to 70° C.

The withdrawal temperature of the crude acrylic acid (stream 5) isusually from 80 to 110° C. The crude acrylic acid is cooled to from 14to 20° C. and, if desired, fed into a crystallizer. The mother liquorfrom such a crystallization is prewarmed to from 85 to 95° C. and fedback into the column (stream 6). It will be appreciated that the heatfrom stream 5 may be used to prewarm stream 6 (integrated energysystem). Particular preference is given to using the already cooledstream 5 to melt the acrylic acid crystals obtained in the crystallizer.The heat exchangers used for this purpose are subject to norestrictions.

The recycling temperature of the dilute acid (stream 8) into the columnis generally from 20 to 35° C.

The columns usable for fractional condensation are subject to noparticular restrictions. In principle, all columns having separatinginternals are suitable.

The column comprises at least one cooling device. For this purpose, allheat transferors or heat exchangers which indirectly (externally) removethe heat liberated during condensation are suitable. For this purpose,all conventional apparatus may be used, and preference is given to usingtube bundle heat exchangers, plate heat exchangers and air coolers.Useful coolants for air coolers are correspondingly air and for furthercooling devices are cooling liquids, in particular water. When only onecooling device is provided, it is preferably installed at the top of thecolumn where the low-boiler fraction is condensed. Those skilled in theart can easily determine the number of cooling devices requireddepending on the desired purity of the condensed fractions and hence ofthe components, while the purity of the condensed components issubstantially determined by the installed separation performance of thecolumn, i.e. the column height, diameter and the energy introduced viathe gas mixture to be condensed. When a plurality of cooling devices arepresent, they are conveniently installed in different sections ofcolumn. For example, a hot gas mixture which, as well as the highfraction of noncondensable components, comprises at least onehigh-boiler, at least one medium-boiler and at least one low-boilerfraction may be provided with a cooling device in the lower section ofthe column to condense the high-boiler fraction and a cooling device atthe top of the column to condense the low-boiler fraction. The condensedfractions are withdrawn from the respective sections of the column,preferably via sidestream takeoffs or collecting trays. Depending on thenumber of components in the high-boiler, medium-boiler and low-boilerfractions, a plurality of sidestream takeoffs may be provided. Thefractions withdrawn via the sidestream takeoffs may then be subjected tofurther purification steps, for example, distillative or extractiveseparating processes or crystallization, depending on the nature of thesecondary components and desired purity of the components. In apreferred embodiment of the invention, a high-boiler takeoff, alow-boiler takeoff and 1 or 2 medium-boiler takeoffs are provided.

The pressure present in the column depends on the quantity ofnoncondensable components and is generally from 0.5–5 bar absolute,frequently from 0.5–3 bar absolute and in many cases from 0.5–2 barabsolute. The exact operating conditions for the column, such astemperature and pressure, connection and arrangement of the coolingdevice or devices, arrangement of the sidestream takeoffs forwithdrawing the desired fractions, the choice of column height andcolumn diameter, the number and spacing of the separatinginternals/trays in the column or the type of the separating columninternals, may be determined by those skilled in the art on the basis ofexperiments customary in the field and depending on the separation task.

The sidestream takeoff for the high-boiler fraction is mounted at thelowermost collecting tray, whose design is not restricted, of thecolumn. If desired, a plurality of collecting trays may also be used fora plurality of high-boiler takeoffs which, with the exception of thelowermost collecting tray, may have suitable overflow devices.

The process according to the invention is advantageously carried out inthe presence of stabilizers of acrylic acid which are known per se.

For the purposes of this document, stabilizers are such compounds whichdelay and/or inhibit the polymerization of acrylic acid.

Examples of useful stabilizers include phenolic compounds, amines, nitrocompounds, phosphorus or sulfur compounds, hydroxylamines, N-oxyls andcertain inorganic salts, and also possibly mixtures thereof in thepresence or absence of molecular oxygen.

Preference is given to stabilizers such as phenothiazine, N-oxyls orphenolic compounds.

N-oxyls (nitroxyl radicals or N-oxyl radicals, compounds which have atleast one >N—O. -group) include, for example,4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl,4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl,4-acetoxy-2,2,6,6-tetramethylpiperidine-N-oxyl,2,2,6,6-tetramethylpiperidine-N-oxyl or3-oxo-2,2,5,5-tetramethylpyrrolidine-N-oxyl.

Examples of phenolic compounds include alkylphenols, for example o-, m-or p-cresol (methylphenol), 2-tert-butyl-4-methylphenol,6-tert-butyl-2,4-dimethylphenol, 2,6-di-tert-butyl-4-methylphenol,2-tert-butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol,2-methyl-4-tert-butylphenol, 4-tert-butyl-2,6-dimethylphenol or2,2′-methylenebis(6-tert-butyl-4-methylphenol), 4,4′-oxydiphenyl,3,4-methylenedioxydiphenol (sesamol), 3,4-dimethylphenol, hydroquinone,pyrocatechol (1,2-dihydroxybenzene),2-(1′-methylcyclohex-1′-yl)-4,6-dimethylphenol, 2- or4-(1′-phenyleth-1′-yl)phenol, 2-tert-butyl-6-methylphenol,2,4,6-tris-tert-butylphenol, 2,6-di-tert-butylphenol,2,4-di-tert-butylphenol, 4-tert-butylphenol, nonylphenol [11066-49-2],octylphenol [140-66-9], 2,6-dimethylphenol, bisphenol A, bisphenol F,bisphenol B, bisphenol C, bisphenol S, 3,3′,5,5′-tetrabromobisphenol A,2,6-di-tert-butyl-p-cresol, Koresin® from BASF AG, methyl3,5-di-tert-butyl-4-hydroxybenzoate, 4-tert-butylpyrocatechol,2-hydroxybenzyl alcohol, 2-methoxy-4-methylphenol,2,3,6-trimethylphenol, 2,4,5-trimethylphenol, 2,4,6-trimethylphenol,2-isopropylphenol, 4-isopropylphenol, 6-isopropyl-m-cresol,n-octadecyl-β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate,1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane,1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene,1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate,1,3,5-tris(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxyethylisocyanurate, 1,3,5-tris(2,6-dimethyl-3-hydroxy-4-tert-butylbenzyl)isocyanurate or pentaerythrityltetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate],2,6-di-tert-butyl-4-dimethylaminomethylphenol,6-sec-butyl-2,4-dinitrophenol, Irganox® 565, 1141, 1192, 1222 and 1425from Ciba Spezialitätenchemie, octadecyl3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate, hexadecyl3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate, octyl3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate,3-thia-1,5-pentanediolbis[(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate],0,4,8-dioxa-1,11-undecanediolbis[(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate],4,8-dioxa-1,11-undecanediolbis[(3′-tert-butyl-4′-hydroxy-5′-methylphenyl)propionate],1,9-nonanediol bis[(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionate],1,7-heptanediaminebis[3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)-propionamide],1,1-methanediaminebis[3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)-propionamide],3-(3′,5′-di-tert-butyl-4′-hydroxyphenyl)propionic acid hydrazide,3-(3′,5′-dimethyl-4′-hydroxyphenyl)propionic acid hydrazide,bis(3-tert-butyl-5-ethyl-2-hydroxyphen-1-yl)methane,bis(3,5-di-tert-butyl-4-hydroxyphen-1-yl)methane,bis[3-(1′-methylcyclohex-1′-yl)-5-methyl-2-hydroxyphen-1-yl]-methane,bis(3-tert-butyl-2-hydroxy-5-methylphen-1-yl)methane,1,1-bis(5-tert-butyl-4-hydroxy-2-methylphen-1-yl)ethane,bis(5-tert-butyl-4-hydroxy-2-methylphen-1-yl)sulfide,bis(3-tert-butyl-2-hydroxy-5-methylphen-1-yl)sulfide,1,1-bis(3,4-dimethyl-2-hydroxyphen-1-yl)-2-methylpropane,1,1-bis(5-tert-butyl-3-methyl-2-hydroxyphen-1-yl)butane,1,3,5-tris[1′-(3″,5″-di-tert-butyl-4″-hydroxyphen-1″-yl)meth-1′-yl]-2,4,6-trimethylbenzene,1,1,4-tris(5′-tert-butyl-4′-hydroxy-2′-methylphen-1′-yl)butane,aminophenols, e.g. para-aminophenol, nitrosophenols, e.g.para-nitrosophenol or p-nitroso-o-cresol, alkoxyphenols, for example2-methoxyphenol (guajacol, pyrocatechol monomethyl ether),2-ethoxyphenol, 2-isopropoxyphenol, 4-methoxyphenol (hydroquinonemonomethyl ether), mono- or di-tert-butyl-4-methoxyphenol,3,5-di-tert-butyl-4-hydroxyanisole, 3-hydroxy-4-methoxybenzyl alcohol,2,5-dimethoxy-4-hydroxybenzyl alcohol (syringa alcohol),4-hydroxy-3-methoxybenzaldehyde (vanillin),4-hydroxy-3-ethoxybenzaldehyde (ethylvanillin),3-hydroxy-4-methoxybenzaldehyde (isovanillin),1-(4-hydroxy-3-methoxyphenyl)ethanone (acetovanillone), eugenol,dihydroeugenol or isoeugenol, tocopherols, e.g. α-, β-, γ-, δ- andε-tocopherol, tocol, α-tocopherolhydroquinone, and2,3-dihydro-2,2-dimethyl-7-hydroxybenzofuran(2,2-dimethyl-7-hydroxycoumarane), quinones and hydroquinones, such ashydroquinone, 2,5-di-tert-butyl hydroquinone, 2-methyl-p-hydroquinone,2,3-dimethylhydroquinone, trimethylhydroquinone, 4-methylpyrocatechol,tert-butylhydroquinone, 3-methylpyrocatechol, benzoquinone,2-methyl-p-hydroquinone, 2,3-dimethylhydroquinone,trimethylhydroquinone, 3-methylpyrocatechol, 4-methylpyrocatechol,tert-butylhydroquinone, 4-ethoxyphenol, 4-butoxyphenol, hydroquinonemonobenzyl ether, p-phenoxyphenol, 2-methylhydroquinone,2,5-di-tert-butylhydroquinone, tetramethyl-p-benzoquinone,diethyl-1,4-cyclohexanedione-2,5-dicarboxylate, phenyl-p-benzoquinone,2,5-dimethyl-3-benzyl-p-benzoquinone,2-isopropyl-5-methyl-p-benzoquinone (thymoquinone),2,6-diisopropyl-p-benzoquinone, 2,5-dimethyl-3-hydroxy-p-benzoquinone,2,5-dihydroxy-p-benzoquinone, embelin, tetrahydroxy-p-benzoquinone,2,5-dimethoxy-1,4-benzoquinone, 2-amino-5-methyl-p-benzoquinone,2,5-bisphenylamino-1,4-benzoquinone, 5,8-dihydroxy-1,4-naphthoquinone,2-anilino-1,4-naphthoquinone, anthraquinone, N,N-dimethylindoaniline,N,N-diphenyl-p-benzoquinonediimine, 1,4-benzoquinonedioxime,coerulignone, 3,3′-di-tert-butyl-5,5′-dimethyldiphenoquinone, p-rosolicacid (aurin), 2,6-di-tert-butyl-4-benzylidenebenzoquinone,2,5-di-tert-amylhydroquinone.

An example of an aromatic amine is N,N-diphenylamine and an example of aphenylenediamine is N,N′-dialkylparaphenylenediamine, where the alkylradicals may each independently contain from 1 to 4 carbon atoms and bestraight-chain or branched, an example of a hydroxylamine isN,N-diethylhydroxylamine, examples of phosphorus compounds includetriphenylphosphine, triphenyl phosphite or triethyl phosphite, anexample of a sulfur compound is diphenyl sulfide and examples ofinorganic salts include perchloride, dithiocarbamate, sulfate,salicylate and acetate salts of copper, anganese, cerium, nickel andchromium.

Preference is given to phenothiazine, p-aminophenol, p-nitrosophenol,2-tert-butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol,2-methyl-4-tert-butylphenol, 4-tert-butyl-2,6-dimethylphenol,hydroquinone or hydroquinone monomethyl ether and also manganese(II)acetate, cerium(III) carbonate and cerium(III) acetate, and particularpreference is given to phenothiazine, p-aminophenol, p-nitrosophenol,2-tert-butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol,2-methyl-4-tert-butylphenol, 4-tert-butyl-2,6-dimethylphenol,hydroquinone or hydroquinone monomethyl ether.

Very particular preference is given to hydroquinone monomethyl ether anda mixture of hydroquinone monomethyl ether and phenothiazine.

The way in which the stabilizer is added is not restricted. Each addedstabilizer may be added individually or as a mixture, in liquid form orin dissolved form in a suitable solvent, and the solvent itself may be astabilizer.

The stabilizer may, for example, be added in a suitable formulation atany desired point in the column, to an external cooling circuit or to asuitable reflux stream. Preference is given to adding it directly intothe column or into an external cooling circuit.

When a mixture of a plurality of stabilizers is used, these may beintroduced independently at different metering points, which werementioned above, or the same metering point.

When a mixture of a plurality of stabilizers is used, these may also bedissolved independently in different solvents.

The concentration of the stabilizer in the column for each individualsubstance may be from 1 to 10000 ppm, preferably from 10 to 5000 ppm,more preferably from 30 to 2500 ppm and in particular from 50 to 1500ppm.

Preference is given to stabilizing the column in the following manner:

The upper column region is stabilized by at least one phenolic compound,preferably hydroquinone monomethyl ether. Stabilizer is added accordingto the invention as a melt into the upper part of column section I.c(FIG. 1, stream 12), preferably at the uppermost tray, and/or into thequench circuit I.d/II (not shown) at a concentration of from 50–2500ppm, preferably from 200–1500 ppm, based on the stream in which thestabilizer is metered in.

Preference is given to a stabilizer mixture in which the melt of atleast one stabilizer having a low melting point of, for example, below120° C., preferably below 100° C., more preferably below 80° C. and inparticular below 60° C. serves as solvent for at least one stabilizerhaving a high melting point of, for example, above 120° C., preferablyabove 140° C., more preferably above 160° C. and in particular above180° C.

Particular preference is given to dissolving at least one furtherstabilizer, more preferably phenothiazine, in the melt in aconcentration of from 1–20% by weight, preferably from 5–10% by weight,based on the melt.

It will be appreciated that water-soluble stabilizers may additionallybe metered separately into the upper portion of column section I.c orinto the quench circuit I.d/II as aqueous solutions.

The remaining column region may be stabilized by a from 0.1–1.5% byweight solution of phenothiazine in acrylic acid, and preference isgiven to adding it in the column region where the acrylic acidconcentration is from 5–20%, preferably from 12–18%, and the waterconcentration is from 40–95%, preferably from 40 to 60% (stream 13). Thequantity added is determined in such a way that the phenothiazinecontent in the acrylic acid fraction (medium-boiler fraction, stream 5)is from 10–1000 ppm, preferably from 50–500 ppm.

The “upper column region” mentioned is the region above the columnregion where the acrylic acid and water are present in theconcentrations mentioned.

The stabilizer solutions or stabilizer melts may be metered in usingpumps. For stabilizer melts, preference is given to pressurizedreceivers. For instance, a hydroquinone monomethyl ether/phenothiazinemelt may be forced directly from the melting and receiving vessel via aregulating valve into the column. To this end, a suitable gas, forexample a nitrogen-containing gas, such as nitrogen, air or air/nitrogenmixtures, is injected into the vessel and pressurized to the desiredpressure. Preference is given to pressures of from 2 to 10 bar.Particular preference is given to using a buffer vessel which takes overthe supply of the stabilizer when the melting and receiving vessel isfilled. The vessels, valves, pumps or pipes may each be trace heated, inorder to prevent solidification of the melt.

The quench IV in which the hot product gas mixture of the oxidationreaction is cooled to from 100–180° C. generally requires no additionalstabilization.

The surfaces in the column which are sparingly wetted are generallysprayed with stabilizing liquid. Preference is given to spraying in aportion of liquid withdrawn from the collecting trays (for example,stream 5) above the collecting trays to provide additional wettingthereof. Particular preference is given to spraying a portion of thereflux below the collecting trays against their undersides for wettingof the same.

It has to be considered surprising that phenolic compounds, particularlyhydroquinone monomethyl ether, are a good solvent for phenothiazine. Upto a melting temperature of 100° C., up to about 20% of phenothiazinemay be dissolved. Despite the low solubility of phenothiazine in diluteacid, the stabilization according to the invention does not lead tosolid deposits or blockages by phenothiazine in the upper column region.

The invention also provides melts comprising

-   -   a) at least one phenolic compound,    -   b) phenothiazine and    -   c) optionally at least one further compound which is effective        as a stabilizer.

Typical melts according to the invention have the following composition:

-   -   a): 60–99% by weight, preferably 80–95% and more preferably        90–95%,    -   b): 1–20% by weight, preferably 5–15% and more preferably 5–10%        and    -   c): 0–20% by weight, preferably 0–15%, more preferably 0–5% and        most preferably 0%,        where the sum thereof is always equal to 100% by weight.

Preference is given to such melts where c) is selected from the groupconsisting of the above-recited N-oxyl compounds and inorganic salts,more preferably the N-oxyl compounds.

Particular preference is given to such melts where the phenoliccompounds a) are selected from the group consisting of the above-recitedphenolic compounds, most preferably from the group consisting ofp-aminophenol, p-nitrosophenol, 2-tert-butylphenol, 4-tert-butylphenol,2,4-di-tert-butylphenol, 2-methyl-4-tert-butylphenol,4-tert-butyl-2,6-dimethylphenol, hydroquinone and hydroquinonemonomethyl ether. In particular, compound a) is hydroquinone monomethylether.

The melting point of the melts according to the invention is generally100° C. or lower, and preference is given to those having a meltingpoint of 80° C. or lower and very particular preference to those havinga melting point of 60° C. or lower.

The invention also provides the use of the melt according to theinvention for stabilizing ethylenically unsaturated compounds inprocesses for preparing them, for example, styrene, acrylonitrile, vinylacetate, vinyl chloride, butadiene, isoprene or chloroprene, preferablyα,β-ethylenically unsaturated carbonyl compounds such as acrylic acid,methacrylic acid, acrolein, methacrolein, crotonic acid, maleic acid,maleic anhydride, methyl methacrylate, methyl acrylate, ethyl acrylate,butyl acrylate or 2-ethylhexyl acrylate, more preferably acrylic acid,methacrylic acid, acrolein or maleic anhydride, most preferably acrylicacid or methacrylic acid, and in particular acrylic acid.

The process according to the invention facilitates a more economicallyviable workup of acrylic acid by reducing the effort associated with thepreparation of the stabilizer batches and also improves stabilization inthe upper column region, which leads to a higher yield and longerrunning time of the column and accordingly reduced downtime.

The present invention further provides a process for rectificativelyseparating substance mixtures comprising at least one polymerizablecompound in the presence of a stabilizer composition comprising at leastone phenolic stabilizer, wherein the stabilizer composition is meteredinto the rectification unit as a melt.

Polymerizable compounds are ethylenically unsaturated compounds,preferably those ethylenically unsaturated compounds which canpolymerize by a free radical polymerization mechanism. Examples includeesters of (meth)acrylic acid with alcohols which have from 1 to 20carbon atoms, for example methyl(meth)acrylate, ethyl(meth)acrylate,n-butyl(meth)acrylate and 2-ethylhexyl (meth)acrylate, vinylaromaticcompounds, for example styrene, divinylbenzene, α,β-unsaturatednitriles, for example acrylonitrile and methacrylonitrile,α,β-ethylenically unsaturated aldehydes, for example acrolein andmethacrolein, vinyl esters, for example vinyl acetate and vinylpropionate, halogenated ethylenically unsaturated compounds, for examplevinyl chloride and vinylidene chloride, conjugated unsaturatedcompounds, for example butadiene, isoprene and chloroprene,monounsaturated compounds, for example ethylene, propylene, 1-butene,2-butene and isobutene, cyclic monounsaturated compounds, for examplecyclopentene, cyclohexene and cyclododecene, N-vinylformamide,allylacetic acid, vinylacetic acid, monoethylenically unsaturatedcarboxylic acids having from 3 to 8 carbon atoms and also theirwater-soluble alkali metal, alkaline earth metal or ammonium salts, forexample: acrylic acid, methacrylic acid, dimethacrylic acid, ethacrylicacid, maleic acid, citraconic acid, methylmalonic acid, crotonic acid,fumaric acid, mesaconic acid and itaconic acid, maleic acid,N-vinylpyrrolidone, N-vinyllactams, for example N-vinylcaprolactam,N-vinyl-N-alkyl-carboxamides or N-vinylcarboxamides, for exampleN-vinylacetamide, N-vinyl-N-methylformamide andN-vinyl-N-methylacetamide, vinyl ethers, for example methyl vinyl ether,ethyl vinyl ether, n-propyl vinyl ether, isopropyl vinyl ether, n-butylvinyl ether, sec-butyl vinyl ether, isobutyl vinyl ether, tert-butylvinyl ether and 4-hydroxybutyl vinyl ether, and vinylphosphonic acid andalso mixtures thereof.

Preference is given to (meth)acrylic esters, vinylaromatic compounds,halogenated ethylenically unsaturated compounds, monoethylenicallyunsaturated carboxylic acids and vinyl ethers, particular preference to(meth)acrylic esters, vinylaromatic compounds and monoethylenicallyunsaturated carboxylic acids, very particular preference to(meth)acrylic esters and monoethylenically unsaturated carboxylic acidsand in particular to monoethylenically unsaturated carboxylic acids.

Substance mixtures comprising polymerizable components are those inwhich at least 50% by weight of the components which are liquid at roomtemperature (25° C.) are polymerizable components, preferably at least60% by weight, more preferably at least 75% by weight, most preferablyat least 85% by weight and in particular at least 90% by weight.

Rectificative separation may mean a fractional condensation of asubstantially gaseous feed or a distillation of a substantially liquidfeed. The rectificative separation may be effected in a separatingcolumn having, for example, from 1 to 150 theoretical plates, preferablyfrom 2 to 120, more preferably from 5 to 100, most preferably from 10 to80 and in particular from 20 to 50, theoretical plates.

The rectification units are of designs known per se and have thecustomary internals. Useful column internals include in principle anycommon internals, for example trays, structured packings and/or dumpedpackings. Among the trays, preference is given to model cap trays, sievetrays, valve trays, Thormann trays and/or dual-flow trays, and among thedumped packings, preference is given to those having rings, spirals,saddles, Raschig rings, Intos rings or Pall rings, barrel or Intaloxsaddles, Top-Pak, etc., or braids.

The rectification units generally have at least one condenser ofcustomary design, for example a direct or indirect condenser, preferablya tube or plate heat exchanger or quench cooler, and distillation unitsadditionally have at least one evaporator of customary design, forexample a tube bundle, plate, thin-film or falling-film evaporator.

The bottom and top temperatures and pressures depend on thepolymerizable compound to be separated. In general, operation iseffected at reduced pressure in order to reduce the distillationtemperature, but rectification may also be carried out at elevatedpressure in the case of volatile compounds.

Examples of top temperatures and pressures are given for the followingpolymerizable compounds:

Top temperature [° C.] Top pressure [° C.] Styrene 50–150 50–1013 Vinylacetate 30–80  200–1013  Acrylonitrile Vinyl propionate 50–100 200–1013 Methyl vinyl ether −30–+10  500–2000  Ethyl vinyl ether 0–40 500–2000 4-Hydroxybutyl vinyl 80–190 50–1013 ether Methacrylic acid 80–160100–1013  Methyl methacrylate 50–100 100–1013  Acrylic acid 60–14050–1013 Methyl acrylate 30–80  50–1013 Ethyl acrylate 40–100 50–1013Butyl acrylate 70–150 50–1013 2-Ethylhexyl 80–200 50–1013 acrylate

Depending on the number of theoretical plates, the bottom temperature ineach case will generally be from 10 to 50° C. higher than the toptemperature.

Stabilizer compositions are those in which at least 50% by weight of thecomposition is active as a stabilizer against polymerization, preferablyfree radical polymerization, of the polymerizable compound to bestabilized, preferably at least 65% by weight, more preferably at least75% by weight, most preferably at least 90% by weight and in particular100% by weight.

The melting point of such a stabilizer composition is preferably above0° C. and more preferably above 250° C. The melting point of such astabilizer composition is, for example, below 100° C., preferably below80° C. and more preferably below 600° C.

The stabilizer compositions comprise, for example, at least one of theabove-cited phenolic compounds, preferably p-aminophenol,p-nitrosophenol, 2-tert-butylphenol, 4-tert-butylphenol,2,4-di-tert-butylphenol, 2-methyl-4-tert-butylphenol,4-tert-butyl-2,6-dimethylphenol, hydroquinone or hydroquinone monomethylether, more preferably 2-tert-butylphenol, 4-tert-butylphenol,2,4-di-tert-butylphenol, 2-methyl-4-tert-butylphenol,4-tert-2,6-dimethylphenol, hydroquinone or hydroquinone monomethylether, and the stabilizer compositions most preferably comprise one ofthe above-cited melts according to the invention, and in particular, thestabilizer composition is one of the above-cited melts according to theinvention.

The stabilizer composition may be metered in as a melt at any desiredpoint in the rectification unit, including the lines and devicesconnected to it, for example condensers, evaporators or vacuum units,and preference is given to metering it in to the upper half (based onthe number of separating internals) of the column, particular preferenceto metering it into the upper third, very particular preference tometering it into the upper quarter and in particular into the uppertenth, especially onto the uppermost tray. In a further preferredembodiment, the molten stabilizer composition is added to the refluxstream and fed with it into the upper section of the column.

The ppm and percentage values reported in this document refer, unlessotherwise stated, to ppm by weight and percentage by weight.

The process according to the invention is illustrated by the followingexample:

INVENTIVE EXAMPLE (FIG. 5)

The heterogeneously catalyzed gas phase oxidation gave a product mixture(stream 1) at 270° C. having the following composition:

 11.5% by weight of acrylic acid  0.28% by weight of acetic acid 27 ppmby weight of propionic acid 0.093% by weight of maleic anhydride  0.1%by weight of acrolein  0.1% by weight of formaldehyde 31 ppm by weightof furfural 25 ppm by weight of benzaldehyde  0.29% by weight of propene 3.8% by weight of oxygen  5.2% by weight of water  2.8% by weight ofcarbon oxides, and the remainder N₂

The product mixture (3600 g/h) was cooled in a spray cooler (IV) to atemperature of 121° C. The sprayer liquid used was the high-boilerfraction (stream 3) withdrawn from the separating column via acollecting tray. The sprayer liquid was circulated through the tubebundle heat exchanger (III) operated with heat-transfer oil. 43 g/h oflow-boilers were continuously withdrawn from the circuit (stream 4).

The product mixture (3600 g/h) was cooled in a spray cooler (IV) to atemperature of 121° C. The sprayer liquid used was the high-boilerfraction (stream 3) withdrawn from the separating column via acollecting tray. The sprayer liquid was circulated through the tubebundle heat exchanger (III) operated with heat-transfer oil. 43 g/h oflow-boilers were continuously withdrawn from the circuit (stream 4).

The high-boilers (stream 4) were collected. The oligomeric acrylic acidscontained therein were dissociated batchwise in a stirred vessel to giveproducts of value (not shown). The dissociation was carried out at 190°C. and a pressure of 500 mbar. The dissociation residue was diluted with25% by weight of methanol and disposed of. The dissociation distillatewas collected and continuously introduced into the quench circuit(III/IV). 34 g/h of dissociation distillate were recycled. Thedissociation distillate comprised from 0.5–1.0% by weight ofhydroquinone monomethyl ether and did not have to be additionallystabilized.

The product gas mixture cooled to a temperature of 121° C. wasintroduced into the separating column below the collecting tray (columnregion Ia).

The column was a tray column having 45 dual flow and 40 bubble captrays. The tray above tray 15 was configured as a further collectingtray. 1680 g/h of crude acrylic acid at a temperature of 101° C. weredischarged via this tray and had the following composition (stream 5):

acrylic acid   97% by weight acetic acid  0.6% by weight propionic acid640 ppm by weight furfural  0.4% by weight maleic anhydride 0.14% byweight benzaldehyde 550 ppm by weight water  1.5% by weight

The crude acrylic acid was introduced to a suspension crystallizer.

In addition, 620 g/h of crude acrylic acid were introduced as anadditional reflux to tray 15.

At the top of the column, a gaseous mixture was withdrawn and subjectedin spray cooler (V) to partial condensation. 482 g/h of the resultingdilute acid which consists essentially of 5.5% by weight of acrylicacid, 5.2% by weight of acetic acid and 85% by weight of water wererecycled into the top of the column at a temperature of 30° C. (stream9). 114 g/h of the dilute acid were continuously withdrawn (stream 10).

A solution of 5% by weight of phenothiazine in hydroquinone monomethylether was introduced as a melt (at a temperature of 60° C.) into thequench circuit at a rate of 0.5 g/h (stream 12).

A solution of 0.5% by weight of the phenothiazine in acrylic acid wasintroduced at a rate of 18 g/h to the 47^(th) tray of the separatingcolumn (stream 13).

The crystallizer was a stirred vessel (3 l capacity) equipped with ahelical stirrer. The heat of crystallization was removed via the jacketof the vessel. The equilibrium temperature of the solution was 9.7° C.The suspension resulting from the crystallization (solids content about30% by weight) was separated batchwise into crystals and mother liquoron a centrifuge at 2000 rpm (centrifuge diameter 300 mm) and a spinningtime of 1 min. The crystals were then washed with melted (previouslywashed) crystals (134 g/h) for 1 min at 2000 rpm. The mother liquortogether with the washing liquid was recycled to the 15^(th) tray of theseparating column (stream 6).

The analysis of the washed crystals (537 g/h) gave the followingcomposition:

acrylic acid 99.7% by weight acetic acid 0.14% by weight propionic acid230 ppm by weight maleic anhydride  72 ppm by weight furfural 210 ppm byweight benzaldehyde  28 ppm by weight water 0.11% by weight

Even after a running time of 1200 hours, the separating device describedshowed negligible polymer formation.

COMPARATIVE EXAMPLE

Example 2 of the German patent application 100 53 086.9 of 26.10.2000was repeated. After a running time of 1200 hours, the process was shutdown and distinct deposits owing to polymer formation were noticeable.

1. A process for the fractional condensation of a hot gas mixturecomprising acrylic acid and at least one further condensable componentin a column in the presence of at least one stabilizer, which comprisesmetering in at least a portion of the at least one stabilizer as a melt.2. A process as claimed in claim 1, wherein the melt of at least onestabilizer having a melting point below 120° C. is used as a solvent forat least one stabilizer having a melting point above 120° C.
 3. A meltcomprising a) at least one phenolic compound, and b) phenothiazine,wherein said melt has a melting point of greater than 250° C.
 4. Themelt as claimed in claim 3, wherein a) is selected from the groupconsisting of p-aminophenol, p-nitrosophenol, 2-tert-butylphenol,4-tert-butylphenol, 2,4-di-tert-butylphenol,2-methyl-4-tert-butylphenol, 4-tert-butyl-2,6-dimethylphenol,hydroquinone and hydroquinone monomethyl ether.
 5. The melt as claimedin claim 3 which comprises the following composition: a): 60–99% byweight, and b): 1–20% by weight wherein the sum thereof is equal to 100%by weight.
 6. A process as claimed in claim 1, wherein said meteringcomprises introducing the melt comprising a) at least one phenoliccompound, b) phenothiazine and c) optionally at least one compound whichis effective as a stabilizer into the upper column region andintroducing phenothiazine into the remaining column region.
 7. A processas claimed in claim 1, wherein the hot gas mixture is cooled in anapparatus isolated from the column.
 8. A process as claimed in claim 1,wherein at least one discharged stream is subjected to a thermal and/orcatalytic treatment.
 9. A process as claimed in claim 1 which is carriedout in the presence of molecular oxygen.
 10. A method of stabilizingethylenically unsaturated compounds in processes for preparing theethylenically unsaturated compounds which comprises incorporating themelt as claimed in claim 3 into a medium comprising the ethylenicallyunsaturated compound.
 11. A process for rectificatively separatingsubstance mixtures comprising at least one polymerizable compound in thepresence of a stabilizer composition comprising at least one phenolicstabilizer, which comprises metering the stabilizer composition into arectification unit as a melt.
 12. The melt as claimed in claim 3,wherein said melt further comprises at least one stabilizer, whereinsaid stabilizer delays or inhibits polymerization of acrylic acid. 13.The melt as claimed in claim 12, wherein said stabilizer is present in aconcentration of up to 20% by weight.
 14. The melt as claimed in claim12, wherein said stabilizer is selected from the group consisting of aphenothiazine, an N-oxyl, and a phenolic compound.
 15. A melt comprisinga) at least one phenolic compound, and b) at least 5% by weigh ofphenothiazine.
 16. The melt as claimed in claim 15, wherein a) isselected from the group consisting of p-aminophenol, p-nitrosophenol,2-tert-butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol,2-methyl-4-tert-butylphenol, 4-tert-butyl-2,6-dimethylphenol,hydroquinone and hydroquinone monomethyl ether.
 17. The melt as claimedin claim 15 which comprises the following composition: a): 60–99% byweight, and b): 5–20% by weight, wherein the sum thereof is equal to100% by weight.
 18. The melt as claimed in claim 15, wherein said meltfurther comprises at least one stabilizer, wherein said stabilizerdelays or inhibits polymerization of acrylic acid.
 19. The melt asclaimed in claim 18, wherein said stabilizer is present in aconcentration of up to 20% by weight.
 20. The melt as claimed in claim18, wherein said stabilizer is selected from the group consisting of aphenothiazine, an N-oxyl, and a phenolic compound.
 21. The melt asclaimed in claim 3, wherein said melt has a melting point of greaterthan 25° C. and less than 100° C.
 22. The melt as claimed in claim 3,wherein said melt has a melting point of greater than 25° C. and lessthan 80° C.
 23. The melt as claimed in claim 3, wherein said melt has amelting point of greater than 25° C. and less than 60° C.